Coating aluminum



Patented July 10, 1934 1.965.683 COATING ALUMINUM Harold K. Work, Oakmont, Pa assignor, by mesne assignments, to Aluminum Colors, Incorporated, Indianapolis, Ind., a corporation of Delaware No Drawing.

Application January 26, 1932, Serial No. 589,057

3 Claims.) (Cl. 204-1) The invention relates to oxide-coated aluminum" by which term is herein defined aluminum and aluminum base alloys coated with a hard coating of appreciable thickness composed in substantial part of aluminum oxide.

Oxide coatings are built up on aluminum surfaces by treating the surface with chemical solutions, either with or without applied external electrical energy. As is known, difliculties may be lit) encountered when aluminum is made the anode of an electrolytic cell containing dibasic organic acid solutions, such as oxalic acid, malonic acid and the like, as-the dibasic organic acid electrolyte tends to severely attack and pit the alu- M minum during electrolysis. Likewise the oxide coating so formed is often yellowish in color and cannot be built up to the desired thickness in an even and uniform manner. On the other hand, coatings produced in a dibasic organic acid are W very hard and have other similarly useful properties. It is also known that when aluminum is made the anode of an electrolytic cell containing sulfuric acid solutions as the electrolyte, the oxide coating produced on aluminum is white to 2355 colorless and during the process of formation there is little, if any, tendency to localized attack or pitting. However, the oxide coatings produced by this method often do not have the hardness required for commercial purposes and .80 are not capable of being built into great thicknesses because of their propensity to soften as treatment progresses.

I have determined that by first treating the aluminum surface as an anode in sulfuric acid $5 solutions and building thereon an oxide coating, I can later build between said oxide coating and the aluminum to which it is attached a second oxide coating, by thereafter treating the aluminum as an anode in a solution of a dibasic organlc acid. The unforseen possibility of building between one oxide coating and the aluminum surface to which it is integrally united, a second oxide coating having. different properties, allows the formation of a multiple-layered oxide coating on aluminum, which coating has modified properties reflecting the properties of its component parts (the sulfuric acid oxide coating and the dibasic organic acid oxide coating) and likewise new properties not had by either. I have further determined that by first forming on the aluminum surface, by means of sulfuric acid,- an oxide layer and later and thereafter forming between said oxide layer and the aluminum surface, by means of oxalic acid, a second oxide Ni layer, I am able to provide, with relative ease,

dibasic organic acid solution electrolytes.

hard, adherent, multiple-layered oxide coatings of great thicknesses and to avoid the difficulties heretofore encountered when singlelaye red oxide coatings of great thickness were desired.

In accordance with my invention, the aluminum is first treated with sulfuric acid solution electrolytes to produce a pervious oxide layer thereon and is then treated with dibasic organic acid solution electrolytes to produce through said pervious oxide coating a second layer of oxide coating. The multiple-layered oxide coating produced on the aluminum does not possess the yellowish tinge of oxalic acid-formed oxide coatings. It is as hard as the oxalic acid-formed oxide coating and considerably harder than any white sulfuric acid coating heretofore produced. It is formed readily and .without the localized pitting or attack usually attendant with the use of The total thickness of the multiple-layered coating may be made to exceed the coating thicknesses heretofore produced by oxide coating processes but the thick multiple-layered coating will, however, retain the hardness, the lightness in color, and the freedom from pitting which have been desirable properties not heretofore combined in one coating.

In the practice of my invention the aluminum is first made the anode in a sulfuric acid solution electrolyte containing from about 2 to '70 per 35 cent by weight of sulfuric acid. The electrolyte is preferably maintained at temperatures below about 40 centigrade; Electrical energy is impressed'upon the cell, the voltage usually being about 6 to 35 volts. This treatment may be of extended or short duration but often,'in connection with the methods of the present invention, is relatively short and does not extend for more than about 15 minutes. The aluminum, now provided with a pervious oxide layer, is made the anode of an electrolytic cell in an electrolyte containing a dibasic organic acid. Of these acids, I prefer oxalic acid or malonic acid, and when using oxalic acid the electrolyte preferably contains about 2 to 9 per cent of that acid by weight. The temperature of the electrolyte is preferably maintained below about 45 centigrade and the voltage of the cell should normally be about 30 to 80 volts. In this case the time of treatment may be extended or short, depending upon the thickness of coating desired. When extremely thick coatings are desired, the treatment may be continued for long periods of time without any of the disadvantages heretofore noted when dibasic organic acid electrolytes have been used. During this process the second oxide layer is formed under the oxide coating initially applied and becomes integrally united with the aluminum surface and with the initially applied oxide coating. As a result the aluminum is coated with a multiple-layered coating of the specific properties above mentioned. g j

When, herein and in the appended claims, it is stated that the aluminum is made the anode, it ,is to be understood that the use of either direct or alternating current is contemplated and that so long as the aluminum is the anode with direct current or an electrode with alternating current, the anodic oxide coating will be produced.

Having thus described my. invention and the manner in which it may be performed, I claim:

1. A method of providing aluminum with a multiple-layered integrally-united oxide coating comprising forming on the aluminum surface a pervious oxide coating by making the aluminum anode in an electrolyte of a 2 to 70 per cent solution of sulfuric acid, thereafter making the aluminum anode in an electrolyte o! a dibasic organic acid of the group consisting of oxalic acid and malonic acid, and forming through the initially applied oxide layer and upon the underlying aluminum surface a second oxide layer of different properties integrally united with said aluminum surface and with the initially applied oxide layer.

2. A method of providing aluminum with amultiple-layered, integrally united oxide coating comprising forming on the aliuninum surface a pervious oxide coating by making the aluminum an anode in an electrolyte of a 2 to '70 per cent solution of sulfuric acid and thereafter making the aluminum an anode in an electrolyte of a 2 to 9 per cent solution of oxalic acid, forming 

